handle notfound error for runtime config

[megan07]

Fixes hashicorp/terraform-provider-google#6740

Release Note Template for Downstream PRs (will be copied)

runtimeconfig: fixed `Requested entity was not found.` error when config was deleted outside of terraform.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=131572"

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccEndpointsService_basic|TestAccFolderIamAuditConfig_multiple|TestAccServiceAccountIamBinding_withAndWithoutCondition|TestAccServiceAccountIamMember_withAndWithoutCondition You can view the result here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=131574"